Flexplates and method for capacitor discharge welding of flexplates

ABSTRACT

A two-piece flexplate assembly having a ring gear welded to a central plate using a capacitor discharge welding process. The weld is established between a joining structure defined between the ring gear and the central plate using either an overlap arrangement, a projection arrangement, or a chamfer arrangement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/736,283 which was filed on Dec. 12, 2012. The entire disclosure ofthe above-noted patent application is incorporated herein by reference.

FIELD

The present disclosure relates generally to two-piece flexplateassemblies for use in motor vehicles and, more particularly, totwo-piece flexplate assemblies having a ring gear welded to a centralplate using a capacitor discharge welding process.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Flexplates, also commonly referred to as flywheels, are used invehicular drivetrain applications to provide a mechanical couplingbetween an output component of an internal combustion engine and aninput component of a transmission. More specifically, one side of theflexplate is secured to an engine crankshaft while the other side of theflexplate is secured to a torque converter. Consequently, the flexplatetransmits engine torque to the torque converter which, in turn,transmits engine torque to the transmission.

As is widely known, the flexplate is configured to deflect axially alimited amount based on changes in engine speed, to accommodate movementand/or vibration of the crankshaft, and in response to fluid pressurevariations inside the torque converter. Additionally, the flexplateserves as an engagement mechanism for the pinion gear of an electricstarter motor. When the electric starter motor receives an electricalcurrent in response to an ignition signal from the vehicle, the piniongear engages and drives a ring gear portion of the flexplate, therebycausing the flexplate to rotatably drive the engine crankshaft. Upon theengine being successfully started, the pinion gear is disengaged whilethe flexplate continues to be rotatably driven by the crankshaft.

In many instances, the flexplate is a two-piece flexplate assemblycomprised of a central plate and a ring gear rigidly secured to an outerrim portion of the central plate. The central plate is generally made(i.e., stamped) of a constant thickness material, although variablethickness materials can also be used. The central plate typicallyincludes a generally flat outer portion and a slightly dished or cantedcentral portion. The central portion includes a center aperture adaptedto receive a crankshaft hub and a plurality of mounting apertures foraligning and mounting the flexplate to the crankshaft. The degree ofdishing is optional and depends primarily upon the space availablebetween the crankshaft and the torque converter. Likewise, a pluralityof mounting apertures extend through the outer portion of the centerplate for mounting the flexplate assembly to the torque converter.

Various methods have been employed to rigidly secure the ring gear tothe central plate of conventional two-piece flexplate assemblies. Forexample, U.S. Publication No. US 2007/0277643 describes a two-pieceflexplate assembly wherein an adhesive is used to rigidly secure thering gear to the central plate. Additionally, U.S. Publication No. US2007/0277644 discloses a two-piece flexplate assembly wherein the ringgear is press-fit and welded to the central plate using a gas metal arcwelding (GMAW) process, commonly referred to as MIG and MAG welding. Itis also known to laser weld the ring gear to the central plate.

Currently, the use of GMAW and laser welding processes are sufficientand acceptable for use in the manufacture of two-piece flexplateassemblies. However, each has certain disadvantages. Specifically, whileGMAW welding is reasonably fast and inexpensive, its use may result inhardness reduction of the ring gear teeth, potential part distortion andhigh residual stress due to the high heat requirements. Likewise, whilelaser welding is a more advanced and precise process, it is inherentlymore expensive and complicated.

The use of these welding processes is further limited in those flexplateassemblies using a nitrided central plate. Nitrided central plates arecommonly used because they permit reduce component weight as well asimprove the yield and tensile strength of the central plate. However, itis difficult to consistently form a good weld between one or morenitrided parts because the weld zone can be very porous due to therelease of nitrogen gas into the weld pool as the nitride layerdecomposes during the welding process. Specifically, due to fastsolidification of the weld pool, the nitrogen gas can become trappedand, as a result, form porous bubbles in the weld zone. To avoid thisundesirable weld porosity issue, current solutions include removing thenitrided layer prior to welding and/or masking the weld surface duringnitriding to establish a naked weld surface. Unfortunately, thesesolutions are not always commercially practical because they add extraprocess steps and costs to the overall production of two-piece flexplateassemblies.

Thus a need exists to develop alternative methods of welding two-pieceflexplate assemblies which address and overcome these disadvantages.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In accordance with one aspect of the present disclosure, a two-pieceflexplate assembly for use in the powertrain of a motor vehicle includesa ring gear that is welded to an outer portion of a central plate usinga capacitor discharge (CD) welding process.

In accordance with a related aspect of the present disclosure, thecentral plate of the two-piece flexplate assembly is nitrided prior tobeing CD welded to the ring gear. During the CD welding process, a highpressured electrical current travels through the weld area for meltingthe materials while simultaneously a large clamping force is applied tothe ring gear and the central plate to join the molten materials. Thecombination of high pressure current and large clamping force acts toimmediately expel or drive out the nitrogen gas, or any other gases,from the weld zone as soon as the gases are formed.

In accordance with these and other aspects, features and advantages ofthe present disclosure, a two-piece flexplate assembly can be CD weldedusing an overlapping joining structure between the ring gear and thecentral plate. In a first exemplary embodiment, a first overlappingjoining structure is established by the central plate having a largerdiameter than the inner diameter of the ring gear so as to define acontinuous overlap therebetween. In a second exemplary embodiment, asecond overlapping joining structure is established by the ring gearhaving a stepped shoulder formed in its inner diameter against which anouter peripheral portion of the central plate is engaged to define acontinuous overlap therebetween. In a third exemplary embodiment, athird overlapping joining structure is established by the ring gearhaving a stepped shoulder formed in its inner diameter against which aplurality of outwardly extending projections formed on the outerperiphery of the central plate engage to define a non-continuous overlaparrangement.

In accordance with further alternative embodiments, a two-pieceflexplate assembly utilizing a projection type joining structure betweenthe ring gear and the central plate is disclosed. In one exemplaryembodiment, the central plate is formed to include a plurality oftransverse projections protruding from one face surface and acorresponding plurality of depressions extending into the other facesurface. The projections rest on and engage a ring portion of the ringgear prior to CD welding to define a “plate projection” arrangement. Ina second exemplary embodiment, the ring gear is machined to form acontoured projection around its inner diameter such that the outerperipheral portion of central plate engages the tip of the contouredprojection prior to CD welding to define a “ring gear projection”arrangement.

In accordance with yet another alternative embodiment, a two-pieceflexplate assembly utilizes a ring gear machined to include a chamfer onits inner diameter surface against which an outer edge portion of thecentral plate rests prior to CD welding to define a “chamfer type”contact arrangement.

Further areas of applicability will become apparent from the descriptionprovided hereon. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a plan view illustrating an exemplary two-piece flexplateassembly.

FIGS. 2-4 illustrate a two-piece flexplate assembly manufactured using acapacity discharge (“CD”) welding process to weld a first overlappingjoining structure established between the ring gear and the centralplate;

FIG. 5 is a partial sectional view of FIG. 4 illustrating the firstoverlapping joining structure in greater detail;

FIGS. 6-8 illustrate a two-piece flexplate assembly manufactured using aCD welding process to weld a second overlapping joining structureestablished between the ring gear and the central plate;

FIG. 9 is a partial sectional view of FIG. 8 illustrating the secondoverlapping joining structure in greater detail;

FIGS. 10-12 illustrate a two-piece flexplate assembly manufactured usinga CD welding process to weld a third overlapping joining structureestablished between the ring gear and the central plate;

FIG. 13 is a partial sectional view of FIG. 12 illustrating the thirdoverlapping joining structure in greater detail;

FIGS. 14-16 illustrate a two-piece flexplate assembly manufactured usinga CD welding process to weld a first projection type joining structureestablished between the ring gear and the central plate;

FIG. 17 is a partial sectional view of FIG. 16 illustrating the firstprojection type joining structure in greater detail;

FIGS. 18-20 illustrate a two-piece flexplate assembly manufactured usinga CD welding process to weld a second projection type joining structureestablished between the ring gear and the central plate;

FIG. 21 is a partial sectional view of FIG. 20 illustrating the secondprojection type joining structure in greater detail;

FIGS. 22-24 illustrate a two-piece flexplate assembly using a CD weldingprocess to interconnect a chamfer type joining structure between thering gear and the central plate;

FIG. 25 is a partial sectional view of FIG. 24 illustrating the chamfertype joining structure in greater detail;

FIG. 26 is a sectioned perspective view of a CD welding tooling unit forCD welding any of the two-piece flexplate assemblies of the presentdisclosure; and

FIG. 27 illustrates a weld section generated by the CD welding processbetween the ring gear and the central plate of a two-piece flexplateassembly.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

Referring to the drawings, wherein like reference numbers indicate likeor corresponding components throughout the several views, a two-pieceflexplate assembly is generally shown at 10. Flexplate assembly 10 isconstructed and manufactured according to the teachings of the subjectinvention and is adapted to be disposed between and provided fordriveably connecting an output component of an internal combustionengine (not shown) to an input component of a transmission (not shown)in a motor vehicle. More specifically, flexplate assembly 10 is adaptedto be mounted to a hub portion of an engine crankshaft via a pluralityof first mounting bolts. Flexplate assembly 10 is further adapted to bemounted to the torque converter via a plurality of second mountingbolts. As is conventional, rotation of the crankshaft causes flexplateassembly 10 to rotate which, in turn, causes the torque converter to berotated in concert with the crankshaft. An electric starter motor istypically used to rotatably drive flexplate assembly 10 during anignition cycle of the motor vehicle to drive the engine crankshaft.

Referring initially to FIG. 1, an exemplary non-limiting structure forflexplate assembly 10 is shown to generally include a central plate 12and a ring gear 14 secured thereto. Central plate 12 is a unitarystamping having an outer portion 16, an inner portion 18 and anintermediate portion 19 therebetween. As will be appreciated, centralplate 12 is rotatably stiff yet axially flexible such that it canresiliently flex to a limited extent to help absorb and transmit forcesexerted upon it during operation of the vehicle's powertrain. Outerportion 16 of central plate 12 includes a plurality of mountingapertures 20 for receiving the second mounting bolts used to secure thetorque converter to flexplate assembly 10. Inner portion 18 of centralplate 12 includes a central aperture 22 for aligning and mountingflexplate assembly 10 to the crankshaft. A plurality of mountingapertures 24 are formed in inner portion 18 of central plate 12 andarranged to receive the first mounting bolts used to secure flexplateassembly 10 to the hub of the crankshaft. Intermediate portion 19 can bestepped or “canted” relative to outer portion 16 and/or inner portion 18to accommodate resilient flexure of central plate 12 during drivenrotation of the crankshaft.

Ring gear 14 is shown to include a rim portion 26 having an outercircumference 28. A plurality of gear teeth 30 are formed in outercircumference 28 of ring gear 14. The number and size of gear teeth 30will vary with the type of engine, size of flexplate assembly 10, andthe type of pinion gear used with the starter motor. While not limitedthereto, intermediate portion 19 of central plate 12 may include steppedsections 34 and 36 relative to planar outer portion 16 and planar innerportion 18. The central plate is made from any suitable material suchas, for example, S.A.E. J1392 that has been coated with a nitride layervia a conventional nitriding process. Ring gear 14 may be made from anysuitable material such as, for example, S.A.E. 1038 to 1055 steel withcarbon, heat-treated to a Rockwell hardness of between 40 and 50.

Referring now to FIGS. 2 through 5, a construction for a flexplateassembly 10A having a basic or first overlapping joint structure betweena ring gear 14A and a central plate 12A for use in association with acapacitor discharge (“CD”) welding process will now be described. Ringgear 14A includes a rim portion 26 defining a planar face surface 40 andan inner diameter surface 42. Central plate 12A includes a stepped orraised flange portion 46 associated with outer portion 16A and which hasa planar face surface 48. Planar face surface 48 of central plate 12Aoverlaps and engages planar surface 40 of ring gear 14A. FIG. 4 bestillustrates that flange portion 46 of central plate 12A defines acontinuous annular overlap joining configuration with respect to rimportion 26 of ring gear 14A. Accordingly, when the CD welding process(to be described hereinafter) is used in association with this firstoverlapping joint structure, a continuous weld seam will be establishedbetween the material associated with nitride flange portion 46 ofcentral plate 12A and the material associated with rim portion 26 ofring gear 14A.

Referring now to FIGS. 6 through 9, a construction for a flexplateassembly 10B having a machined or second overlapping joint structurebetween a ring gear 14B and a central plate 12B for use in associationwith the CD welding process will now be described. Ring gear 14Bincludes a stepped rim portion 50 defining a continuous shoulder flangesurface 52. Central plate 12B includes a planar outer portion 16Bdefining a peripheral edge surface 54 and a planar face surface 56.FIGS. 8 and 9 best illustrate that face surface 56 of central plate 12Brests on shoulder flange surface 52 of ring gear 14B while edge surface54 is located in close proximity to an inner wall surface 58 of steppedrim portion 50. Preferably, ring gear 14B is machined to define innerwall surface 58 and shoulder flange surface 52. Accordingly, when the CDwelding process is used in association with this second overlappingjoint structure, a continuous weld seam will be established between thematerial associated with stepped rim portion 50 of ring gear 14B and thematerial associated with central plate 12B resting thereon.

Referring now to FIGS. 10 through 13, a construction for a flexplateassembly 10C having a non-continuous or third overlapping joiningstructure between a ring gear 14C and a central plate 12C for use withthe CD welding process will now be described. Ring gear 14C isconfigured similar to ring gear 14B and includes a stepped rim portion60 defining a continuous shoulder flange surface 62 and an inner wallsurface 64. An edge portion 66 of central plate 12C is configured to becut-out so as to define a plurality of radially outwardly-extendingprojection tabs 68. FIGS. 12 and 13 illustrate that a planar facesurface 70 of each tab 68 rests on shoulder flange surface 62 while anedge surface 72 of each tab 68 is located in close proximity to innerwall surface 64. Accordingly, when the CD welding process is used inassociation with this third overlapping joint structure, anon-continuous weld seam is established along the periphery.Specifically, since the weld seam is only established between thematerial associated with the projecting tabs 68 on central plate 12C andthe material associated with shoulder flange surface 62 and inner wallsurface 64 of ring gear 14C that are overlapped by tabs 68, a series ofdistinct and spaced weld joints are established.

Referring now to FIGS. 14 through 17, a construction for a flexplateassembly 10D having a “plate” type or first projection joining structurebetween a ring gear 14D and a central plate 12D for use with the CD weldprocess will now be described. FIG. 14 shows ring gear 14D to include arim section 80 defining a planar face surface 82. FIG. 15 shows centralplate 12D to include a series of circumferentially-arranged projections84 extending outwardly from a planar surface 86 of outer portion 16D. Aseries of corresponding depressions 88 extend inwardly from an oppositeplanar surface 90 of outer portion 16D. Projections 84 and depressions88 are concurrently produced via a stamping operation. FIGS. 16 and 17illustrate projections 84 resting on or engaging planar surface 82 ofrim section 80. Accordingly, when the CD welding process is used inassociation with this first projection type joining structure, thematerial associated with projections 84 on central plate 12D and thematerial associated with planar surface 82 engaging projections 84establish a plurality on weld areas. This use of a plurality ofcircumferentially-arranged distinct weld spots permits a higher powerdensity weld compared to a continuous weld. As an option, a series ofdetent chambers 94 can be formed in planar surface 84 of ring gear 14Dwhich are sized and positioned to accept receipt of projections 84 oncentral plate 12D therein. Chambers 94 can be machined into ring gear14D. As a further option, a continuous detent groove 96 can be formed inplanar surface 84 of ring gear 14D which is likewise sized to acceptreceipt of projections 84 thereon.

Referring now to FIGS. 18 through 21, a construction for a flexplateassembly 10E having a “ring gear” type or second projection joiningstructure between a ring gear 14E and a central plate 12E for use inassociation with the CD welding process will now be described. Ring gear14E is machined to define a rim portion 100 having a continuous shaped(i.e., triangular-shaped) projection 102 extending around its innerdiameter. FIG. 19 shows central plate 12E having an annular flange 104formed at its outer peripheral area which defines a planar face surface106. FIGS. 20 and 21 illustrate planar face surface 106 resting on orengaging a tip portion of the triangular-shaped projection 102.Accordingly, when the CD welding process is used in association withthis second projection type joining structure, the material associatedwith projection 102 on ring gear 14E is welded to the materialassociated with the flange 104 on central plate 12E in contact withprojection 102. Projection 102 may be defined by machining a continuousslotted groove into a planar surface 101 of ring gear 14E.

Referring now to FIGS. 22 through 25, a construction for a flexplateassembly 10F having a “continuous chamfer” type joining structurebetween a ring gear 14F and a central plate 12F for use with the CDwelding process will now be described. FIG. 22 shows ring gear 14F toinclude a rim portion 110 having a chamfered edge 112 adjacent its innerdiameter wall surface 114. FIG. 23 shows central plate 12F having acontinuous flange portion 116 associated with its outer portion 16F andhaving a planar face surface 118 and an edge surface 120. FIGS. 24 and25 illustrate edge surface 120 of flange portion 116 on central plate12F engaging chamfered edge 112 of rim portion 110 on ring gear 14F.Accordingly, a continuous weld seam is established upon CD welding ofring gear 14F to central plate 12F.

Referring now to FIG. 26, a tooling unit for a CD welding processcapable of use with each of the various two-piece flexplate assembliespreviously described is shown and identified by 200. A ring gear 14 isfirst loaded onto a first or lower support pad 202. A center pin 204 isprovided at the center of first support pad 202 to assist in centering acentral plate 12 installed thereon. Specifically, central plate 12 islined up by its center hole via centering pin 204 and is rested on topof ring gear 14. As will be understood, a joining structure isestablished by engagement between ring gear 14 and central plate 12. Asecond or upper tool member, such as clamp 206, is specially designedbased on the specific joining structure utilized, to ensure that theelectric current discharged from the capacitor during the CD weldingprocess travels through the intended weld locations on central plate 12.Clamp 206 acts as the top electrode and provides the required clampingpressure, while first support pad 202 acts as the bottom electrode.

Before the capacitor discharge weld cycle commences, clamp 206 is raisedabove installed flexplate assembly 10. Once the cycle is started, clamp206 is lowered. As soon as clamp 206 contacts central plate 12, thestored energy in the capacitor is released while clamping pressure ismaintained. Secondary impulses may be triggered after the primary pulseto reheat the weld area, thereby reducing the heat affect zone coolingrate to present crack formation. Thus, CD welding provides a single andfeasible solution to the different process of welding nitrided centralplates to ring gears to provide a two-piece flexplate assembly.

As noted, during the CD welding process, a high pressure electricalcurrent travels through the weld area to melt the engaged materialswhile simultaneously a large clamping force is applied to join themolten materials. This combination of high pressure current and largeclamping face acts to instantaneously drive the nitrogen gas, generatedby decomposition of the nitrided layer, away from the weld zone. Thiscan be seen from the exemplary weld zone 208 shown in FIG. 27, where thenitrogen gas has been forced out of the heat affect weld area and asolid non-porous weld 210 has been formed between nitride central plate12 and ring gear 14. The weld zone 208 is shown to include areas wheremolten ring gear material 212 is joined with molten central platematerial 214.

CD welding uses energy stored in a capacitor battery which is chargedbetween sequential weld cycles. The actual welding time is relativelyshort, possibly only about 10 milliseconds. Because of this shortwelding time, the energy is concentrated to the weld zone 208 only. Asthe parts cool out rapidly, granulation is relatively fine since theshort cooling phase leaves little time for grain growth. Steel materialscontaining carbon can be welded by reheat pulses which assist inreducing the hardness increase of the heat-affected zone. As noted, CDwelding includes two primary parameters including welding energy andelectrode force.

During welding, the discharged energy pulse causes the contactingportions of the components in the weld zone 208 to melt. The CD weldingprocess allows extremely fast energy release with large peak currents.The heat-affected zone, where the properties of the metal have beenchanged by rapid heating and cooling, is localized to a small areaaround the weld spot.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A flexplate assembly for use with a powertrain ina motor vehicle, the flexplate assembly comprising: a nitrided centralplate having an outer peripheral portion; and a ring gear having a rimportion with gear teeth, said rim portion engaging said outer peripheralportion of said central plate to define a joining structuretherebetween, said joining structure being welded using a capacitordischarge welding operation for establishing a weld seam for rigidlysecuring said central plate to said ring gear.
 2. The flexplate assemblyof claim 1 wherein said joining structure is an overlapping joiningstructure between said ring gear and said central plate, wherein saidrim portion of said ring gear has a planar face surface which overlapsand engages a flange section formed on said outer peripheral portion ofsaid central plate so as to define a continuous overlapping engagementtherebetween, and wherein said weld seam formed by said capacitordischarge welding operation is continuous and completely encircles saidoverlapping joining structure.
 3. The flexplate assembly of claim 1wherein said joining structure is an overlapping joining structurebetween said ring gear and said central plate, wherein said rim portionof said ring gear is stepped to define a continuous shoulder flangesurface and an inner wall surface, wherein said outer peripheral portionof said central plate includes a planar face surface and an edge surfaceconfigured such that said planar face surface engages said shoulderflange surface while said edge surface is located in close proximity tosaid inner wall surface so as to define a continuous overlappingengagement between said ring gear and said central plate, and whereinsaid weld seam formed by said capacitor discharge welding operation iscontinuous and completely encircles said overlapping joining structure.4. The flexplate assembly of claim 3 wherein said stepped portion ofsaid rim portion is machined into said ring gear such that said centralplate is disposed within said ring gear.
 5. The flexplate assembly ofclaim 1 wherein said joining structure is an overlapping joiningstructure between said ring gear and said central plate, wherein saidrim portion of said ring gear is stepped to define a continuous shoulderflange surface and an inner wall surface, wherein said outer peripheralportion of said central plate includes a plurality of radiallyprojecting tabs each defining a planar face surface configured to engagesaid shoulder flange surface and an edge surface configured to belocated in close proximity to said inner wall surface, and wherein saidweld seam formed by said capacitor discharge welding operation isnon-continuous since said weld seam is established between said tabs andcorresponding portions of said shoulder flange surface engaging saidtabs, whereby a series of distinct and spaced weld joints areestablished between said ring gear and said central plate.
 6. Theflexplate assembly of claim 5 wherein said stepped portion of said rimportion is machined into said ring gear such that said central plate isdisposed within said ring gear.
 7. The flexplate assembly of claim 1wherein said joining structure is a projection type joining structurebetween said ring gear and said central plate, wherein said rim portionof said ring gear defines a planar face surface, wherein said outerperipheral portion of said central plate defines a planar engagementsurface having a plurality of projections extendly outwardly therefromand which are configured to engage said planar face surface on said rimportion of said ring gear, and wherein said weld seam formed by saidcapacitor discharge welding operation is a series of weld jointsestablished between said projections and corresponding portions of theplanar face surface said projections.
 8. The flexplate assembly of claim7 wherein said projections are equally-spaced andcircumferentially-aligned on said outer peripheral portion of saidcentral plate.
 9. The flexplate assembly of claim 7 wherein saidprojections are formed in said central plate by a stamping process. 10.The flexplate assembly of claim 7 wherein said planar face surface onsaid rim portion of said ring gear includes a series of detent chamberssized and arranged to receive and retain said projection extending fromsaid central plate.
 11. The flexplate assembly of claim 7 wherein saidplanar face surface of said rim portion includes a detent groove sizedand arranged to receive and retain said projections extending from saidcentral plate thereon.
 12. The flexplate assembly of claim 1 whereinsaid joining structure is a projection type joining structure, whereinsaid rim portion of said ring gear includes a continuous projection,wherein said outer peripheral portion of said central plate includes anannular flange having a planar face surface engaging said continuousprojection on said rim portion of said ring gear, and wherein said weldseam formed by said capacitor discharge welding operation defines acontinuous weld joint established between said continuous projection onsaid ring gear and said planar face surface of said central plate. 13.The flexplate assembly of claim 12 wherein said continuous projectionsurface extends outwardly from a face surface on said rim portion ofsaid ring gear.
 14. The flexplate assembly of claim 12 wherein saidcontinuous projection is formed by a continuous groove machined intosaid rim portion of said ring gear.
 15. The flexplate assembly of claim1 wherein said joining structure is a continuous chamfer type joiningstructure between said ring gear and said central plate, wherein saidrim portion of said ring gear defines a chamfered edge surface, whereinsaid outer peripheral portion of said central plate includes an edgesurface engaging said chamfered edge surface of said rim portion, andwherein said weld seam formed by said capacitor discharge weldingoperation establishes a weld joint between said mating edge surfaces.16. The flexplate assembly of claim 15 wherein said edge surface of saidcentral plate is continuous and completely engages said chamfered edgesurface of said ring gear to define a continuous weld seam.
 17. Theflexplate assembly of claim 15 wherein said edge surface of said centralplate is formed on outwardly projecting tabs such that a series of weldjoints are established between said tabs and corresponding portions ofsaid chamfered edge surface engaging said tabs.
 18. A method ofmanufacturing a two-piece flexplate assembly comprising the steps of:providing a central plate having an outer peripheral portion, saidcentral plate having a nitride layer formed at least on its outerperipheral portion; providing a ring gear having a rim portion; locatingsaid outer peripheral portion of said central plate in engagement with asurface on said rim portion of said ring gear so as to establish ajoining structure therebetween; and welding said ring gear to saidcentral plate along said joining structure using a capacitor dischargewelding process for establishing a weld seam therebetween.
 19. Themethod of claim 18 wherein said joining structure is a continuousoverlapping arrangement such that a continuous weld seam is established.20. The method of claim 18 wherein said joining structure is anon-continuous overlapping arrangement such that said weld seam isdefined by a series of distinct weld joints.
 21. The method of claim 18wherein said joining structure is a projection engagement arrangementwith one of said ring gear and said central plate having a projectionextending outwardly therefrom and engaging a surface portion of theother one of said ring gear and said central plate.